1. Field of the Invention
The present invention relates to a switching power supply device which achieves noise reduction during switching.
2. Description of the Related Art
In a switching power supply device having a switching device connected to the primary of a transformer, such that the switching device is used for a self-excited or separately-excited oscillation to provide an output from the secondary of the transformer, a pulsed voltage across the switching device is provided when the switching device is turned on and off, and noise is radiated externally when the pulse increases and decreases. Radiation noise having a predetermined level or higher of radiation energy might adversely affect external devices, and must be reduced by some technique. A conventional method of effectively reducing the radiation noise is to connect capacitors across both ends of the switching device so as to mitigate a rapid change of the voltage so that high frequency noise components may be eliminated.
FIG. 1 illustrates a conventional switching power supply device of this type, to which capacitors are connected.
In the switching power supply device shown in FIG. 1, a transformer T having the primary winding Np and the secondary winding Ns is connected in series to a first switch circuit S1 and an input power source E. One end of a series circuit comprising a second switch circuit S2 and a capacitor C is connected to one end of the first switch circuit S1. A rectifier smoothing circuit is connected to the secondary winding Ns of the transformer T. The switching power supply device is a self-excited oscillation device. The details of the switching power supply device shown in FIG. 1 are disclosed in Japanese Unexamined Patent Application Publication No. 11-187664. In the switching power supply device disclosed, the first switch circuit S1 includes a first switching device Q1 and a capacitor C1 which are connected in parallel to each other, and the second switch circuit S2 includes a second switching device Q2 and a capacitor C2 which are connected in parallel to each other. The capacitors C1 and C2 allow a rapid change of the voltages generated across the first and second switching devices Q1 and Q2, respectively, to be mitigated, so that high frequency noise components may be eliminated.
However, in the switching power supply device shown in FIG. 1, the capacitors C1 and C2 must have a high voltage rating. Furthermore, the capacitance of the capacitors C1 and C2 must be a predetermined level or higher in order to constantly reduce the radiation noise. Therefore, a problem occurs in that the capacitors C1 and C2 are so large that a compact and low cost switching power supply device may not be achieved.
Accordingly, it is an object of the present invention to provide a switching power supply device which uses a less capacitive impedance to provide higher radiation noise reduction performance.
To this end, the switching power supply device includes a transformer having primary and secondary windings, a first switch circuit, an input power source, a second switch circuit, and a rectifier smoothing circuit connected to the second winding of the transformer. The transformer, the first switch circuit, and the input power source are connected in series. The second switch circuit and a capacitor are connected in series to form a series circuit, and one end of the series circuit is connected to one end of the first switch circuit. The first switch circuit includes a parallel connection circuit comprising a first switching device and a first diode, and the second switch circuit includes a parallel connection circuit comprising a second switching device and a second diode. The switching power supply device further includes a switching control circuit for controlling each of the first and second switching devices so as to be alternately turned on and off with an off-time interposed between on-times. The transformer further has a third winding wound in the same direction as that of the primary winding, and a capacitive impedance element is connected between the termination of the third winding and the input power source.
In FIG. 1, if the primary winding Np of the transformer T has an inductance Lp the resonant frequency fr while the voltage across the first switching device Q1 varies is found by equation (1) as follows:                               f          r                =                  1                      2            ⁢            π            ⁢                                                            L                  p                                ⁢                                  C                  1                                                                                        (        1        )            
where the parasitic capacitance of the first switching device Q1 is negligible.
From equation (1), as the capacitance of the capacitor C1 increases, the resonant frequency fr decreases, and high frequency noise components are reduced.
On the other hand, as shown in FIG. 2, in a power supply device according to the present invention, a transformer T includes a third winding (in FIG. 2, a second driving winding Nb2 having turns a turn Nb2 corresponds to the third winding) which is wound in the same direction as that of the primary winding Np (having turns Np). A capacitive impedance element Ca is connected between the termination of the third winding and an input power source E, so that the capacitors C1 and C2 shown in FIG. 1 may be removed, or, otherwise, may have lower capacitances. If the capacitive impedance is indicated by Ca, and the inductance of the third winding is indicated by La, the resonant frequency fr while the voltage across the first switching device Q1 varies is found by equation (2) as follows.                               f          r                =                  1                      2            ⁢            π            ⁢                                                            (                                                            L                      p                                        +                                          L                      a                                                        )                                ⁢                                  C                  a                                                                                        (        2        )            
It is understood from comparison between equations (1) and (2) that the capacitor Ca can be more compact than the capacitor C1 if the resonant frequencies fr are the same. Therefore, according to the present invention, a capacitive impedance element is connected between the termination of a third winding and an input power source so that the radiation noise caused by a rapid change of the voltage across a switching device can be reduced. This allows the capacitive impedance to be lower, thereby achieving a compact and low-cost switching power supply device.
Preferably, the capacitive impedance element includes a series circuit comprising a capacitor and an inductor.
This prevents a current from rapidly flowing into the capacitor, thereby achieving noise reduction.
The inductor may be a ferrite bead. A compact and low-cost ferrite bead would prevent a current which rapidly flows into the capacitor, in particular, a high frequency current, thereby achieving noise reduction.
Preferably, a voltage generated at the third winding is used to turn on the second switching device. If a driving winding for allowing the second switching device to be turned on is used as the third winding, the transformer may be more compact.
At least one of the first and second switching devices may be a field effect transistor. Therefore, the parasitic diode of the field effect transistor can be used instead of the first and/or second diode. In this case, the first and/or second diode may be removed, thereby making the switching power supply device more compact and light-weight.
Preferably, the transformer includes a driving winding which allows a voltage to turn on the first switching device to be generated to provide self-excited oscillation. Therefore, there is no need for ICs such as oscillation circuits and control circuits, thereby achieving a compact, light-weight, and low-cost switching power supply device.
The transformer may include either a leakage inductor connected between the primary and secondary windings, or an inductor connected in series to the transformer. The resulting inductor and the capacitor form a resonant circuit.
The resonance by the inductor and the capacitor allows the energy accumulated in the inductor to be output without being dissipated, thereby providing high efficiency. Furthermore, the second switching device can perform a zero current turn-off operation, thereby reducing switching loss.
Preferably, the rectifier smoothing circuit includes a diode for rectifying an output of the secondary winding, and a capacitive impedance element connected in parallel to this diode.
The capacitive impedance element connected in parallel to the diode reduces the reverse recovery loss in the diode, thereby providing high efficiency.
The rectifier smoothing circuit may allow energy to be accumulated in the primary winding of the transformer while the first switching device is turned on, and allow the energy to be supplied from the secondary winding of the transformer while the first switching device is turned off.
Therefore, at least one secondary diode connected to the secondary winding of the transformer is only required, thereby achieving a compact, light-weight, and low-cost switching power supply device. However, at least two diodes are necessary if a circuit for rectifying an output at the secondary both while the first switching device is turned on and off is provided.
Accordingly, a more compact capacitor is used to reduce noise which is constantly radiated, and less stress caused by a voltage fluctuation is imposed on the switching device, thereby achieving a compact and low-cost switching power supply device.